1. Field of the Invention
The present invention relates to an optical system having camera shake compensating function, and more specifically, to an optical system (e.g. zoom lens system, fixed focal length lens system) having a camera shake compensating function capable of preventing image blur due to camera shake (e.g. shake of the camera caused when the user holds the camera to perform photographing).
2. Description of the Prior Art
Conventionally, the failure in photographing was mostly attributed to camera shake and being out of focus. In recent years, however, most cameras employ the automatic focusing mechanism and with the improvement of focusing accuracy of the automatic focusing mechanism, the problem of the failure in photographing due to being out of focus has been practically solved. On the other hand, the lens system normally incorporated in the camera has shifted from a fixed focal length lens system to a zoom lens system, and with the shift, the magnification and the longest focal length have been increased. Consequently, camera shake very frequently occurs. As a result, presently, it is no exaggeration to say that the failure in photographing is caused by camera shake. For this reason, a camera shake compensating function is indispensable for the lens system (particularly, zoom lens system).
As optical systems having a camera shake compensating function, optical systems have been proposed in which camera shake is compensated for by decentering a part of the lenses (hereinafter, this type of optical system will be referred to as "camera shake compensating optical system"). For example, Japanese Laid-open Patent Application No. H6-123836 discloses a five unit zoom lens system of positive, negative, negative, positive, negative configuration in which camera shake compensation is made by decentering the negative third lens unit. This five unit zoom lens system is based on a four unit telephoto zoom lens system having positive, negative, positive, negative configuration and the negative second lens unit is divided into two negative lens units. The five unit zoom lens system is compact as a whole since it is based on the four unit system of positive, negative, positive, negative configuration.
In the camera shake compensating optical system, not only optical performance is necessarily excellent in the normal condition (i.e. "pre-decentering condition") but also the generation of aberrations due to the decentering of the lens is necessarily restrained to maintain desired optical performance in the compensated condition (i.e. "post-decentering condition").
However, the five unit zoom lens system of Japanese Laid-open Patent Application No. H6-123836 has a problem that the aberration performance after the camera shake compensation (i.e. after decentering) is inferior. Although Japanese Laid-open Patent Application No. H6-123836 positively appraises the performance of the system at an angle of compensation of approximately 0.15.degree., when the compensation angle is greater, the aberration degradation is impermissible.
In particular, axial coma, which is generated in proportion to the third power of the focal length, increases with an increase in focal length. That is, in a normal telephoto zoom lens system for use in a single-lens reflex camera, the focal length is 120 mm or more (corresponding to a half angle of view of 10.degree. or less) on the longer focal length side in 35 mm format and the minimum F-number is approximately 6.7 or less. When a camera shake compensating system is incorporated therein, since axial coma is proportional to the third power of the focal length and inversely proportional to the second power of the F-number, if the telephoto zoom lens system is provided with a camera shake compensating function, axial coma is very great, so that it is very difficult to excellently correct aberrations (i.e. aberrations including axial coma) with good balance in the entire zoom range.
If a lens unit capable of decentering is added as the first lens unit, aberration performance such as off-axial image point movement error, one-side blur and axial coma after the camera shake compensation can be excellently corrected in the entire zoom range. However, since the first lens unit has a large diameter, a driving mechanism for decentering it should be large. This increases the size of the lens barrel.